Conventionally known image formation methods include forming a thin layer of a toner on a developing roller by means of a magnetic brush. The magnetic brush formed on a developer supporting member supports a two-component developer. Then the toner on the developing roller develops an electrostatic latent image on an electrostatic latent image supporting member.
Developing rollers used in an image-forming apparatuses and having conductive base surfaces covered with resin are known. In one such example, a developing roller has a resin cover layer that contains an electron conductive material and an ion conductive material, and has a volume resistivity (R) ranging from about 107 to about 1010 Ω·cm when the ion conductive material is mixed. The resin cover layer may comprise acrylic resin, epoxy resin, resol resin, and the like.
When small particle diameter toner is used, it is strongly affected by image force and van der Waals force. Therefore, even when a development bias is applied on the developing roller, the small particle diameter toner does not fly to an electrostatic latent image on a photoconductor. This results in a problem with deterioration of developing properties. In order to cause the small particle diameter toner to fly to the electrostatic latent image on the photoconductor, it is necessary to enhance releasability of the toner from the developing roller. In order to increase toner releasability, the developing roller surface may be preferably covered with a resin having superior releasability properties. Examples of such resin include fluorine resin, silicone resin, and the like. However, when a positively charged toner is used, these types of resins will cause a problem because the toner becomes charged-up electrically due to friction. Electrically charged up toner cannot easily fly from the developing roller to the electrostatic latent image on the photoconductor, and deterioration of image density results. Using a urethane resin as the cover resin may suppress such an electric charge-up, but there remains a problem of the urethane resin having unfavorable toner releasability.
Furthermore, covering the surface of the developing roller with a resin creates an additional problem of electric charge accumulation in the resin layer. Dispersing carbon black or a metal-based conductive material in the resin layer is one known method to curb this problem. However, in some cases uneven dispersion of the conductive material within the resin may not prevent the accumulation of electric charge.
There remains a need in the art for a developing device comprising a resin-covered developing roller with improved toner releasability as well as decreased electric charge accumulation in the resin layer.
Citation or identification of any document in the application is not an admission that any such document is available as prior art to the present inventor.